The "Free Online Dictionary of Computing" has the following:
synchronous:
<operating system, communications> 1. Two or more processes
that depend upon the occurrences of specific events such as
common timing signals.
2. Occurring at the same time or at the same rate or with a
regular or predictable time relationship or sequence.
In our case, a synchronous interaction between two or more parties
is one in which the specification of the elements of that interaction
includes references to common clocks or temporal dependencies.